The present invention relates to a DC resistance welding apparatus for resistance-welding workpieces with a direct current, and more particularly to an inverter-type DC resistance welding apparatus for rectifying three-phase AC electric energy into DC electric energy, converting the DC electric energy to high-frequency AC electric energy, then converting the high-frequency AC electric energy again into DC electric energy with an welding transformer and rectifiers, and then supplying the DC electric energy to welding electrodes to weld workpieces.
Resistance welding apparatus include a pair of electrodes for gripping a set of workpiece therebetween. While a welding current is being supplied between the electrodes to generate Joule heat, the electrodes are pressed against the workpieces to weld the workpieces to each other. The resistance welding is highly efficient as it requires no welding rods.
The resistance welding process requires an electric current much greater than other welding processes such as the arc welding process. Therefore, the welding transformer used by the resistance welding process is large and heavy. The large and heavy welding transformer is one of the drawbacks which make it difficult to install the resistance welding apparatus on the arm of a welding robot or the like.
To make the welding transformer smaller in size, there has recently been employed an inverter-type DC resistance welding apparatus which converts DC electric energy to high-frequency AC electric energy, supplies the high-frequency AC electric energy to a welding transformer to lower the voltage thereof, then converts the high-frequency AC electric energy to DC electric energy, and supplies the DC electric energy to welding gun arms. The DC electric energy is first converted to the high-frequency AC electric energy because the high-frequency AC electric energy allows the welding transformer to be relatively small and lightweight since the cross-sectional area of the core of the welding transformer is inversely proportional to the frequency of the high-frequency AC electric energy. The reason for converting the high-frequency AC electric energy back to the DC electric energy for application to the welding gun arms is that the DC electric energy supplied to the welding gun arms avoids a voltage drop which would otherwise be caused by an increased high-frequency impedance due to the stray inductance resulting from the length and shape of the welding gun arms, and also avoids a voltage drop which would otherwise be developed by the skin effect of the welding gun arms, so that the welding apparatus is highly efficient in operation.
One inverter-type DC resistance welding apparatus is shown in FIG. 1 of the accompanying drawings. The DC resistance welding apparatus comprises a converter unit 2, an inverter unit 4, and a welding transformer assembly 6 including a welding transformer 14. Three-phase AC electric energy from a commercial three-phase AC power supply 7 is converted to DC electric energy by a rectifier 8 and a capacitor 10 of the converter unit 2, and the DC electric energy is then converted to AC electric energy having a frequency higher than that of the three-phase AC electric energy by the inverter unit 4 which comprises a bridge of transistors 12a through 12d. The high-frequency AC electric energy is then converted to DC electric energy again by the welding transformer 14 with a central tap 19 on its secondary winding and rectifiers 16a, 16b. The DC electric energy is then supplied between welding electrodes 18a, 18b. The welding electrode 18a is connected to the common joint between the rectifiers 16a, 16b, and the welding electrode 18b is connected to the central tap 19. A pair of workpieces Wa, Wb to be welded together is placed between the welding electrodes 18 a, 18b. When a welding current is passed between the welding electrodes 18a, 18b through the workpieces Wa, Wb, the contacting surfaces of the workpieces Wa, Wb are melted and welded to each other.
DC resistance welding apparatus are always required to supply a large welding current and to be small in size. A large welding current is preferable when welding steel sheets such as plated steel sheets containing materials of different melting points or welding aluminum sheets or the like having a large thermal conductivity.
If the current capacity of the conventional DC resistance welding apparatus is to be increased and the DC resistance welding apparatus is to be installed on the arm of a welding robot, then the welding transformer 14 will be increased in weight, requiring the robot to be large in size, and it is required to connect the transistors 12a through 12d of the inverter unit 4 or the rectifiers 16a, 16b parallel to each other. As a result, the DC resistance welding apparatus may have stability and reliability problems.
Heretofore, the transistors of the inverter are operated within a continuous DC rating range. The inverter is designed such that even if driver circuits connected to the bases of the transistors malfunction and the transistors remain conductive, the inverter will not be broken insofar as it is within its thermal limits. The actual operating range of the inverter is therefore required to be smaller than the continuous DC rating range. As a consequence, the conventional DC resistance welding apparatus is not suitable for use in applications in which a large welding current is to be supplied to the welding electrodes.